1. Field of the Invention
The present invention relates to an apparatus and method for the turbulent mixing of gases. The apparatus comprises a tubular structure having at least two orifices or jets on the internal surface thereof. The orifices or jets are opposed in a manner such that gas streams flowing through these openings into the interior of the tubular structure are mixed in a turbulent manner. In particular, the relative locations of the orifices or jets on the interior surface of the tubular structure provide a swirling flow pattern which is particularly effective in its mixing action.
2. Description of the Background Art
There are numerous requirements for specialized gas mixing apparatus and methods, particularly when a desired gas mixture is not available commercially. Frequently a gas mixture is not available commercially because the gases to be mixed are reactive. It may be the gases have significantly different densities and would separate on standing of the mixture. In the case of reactive gases or gas mixtures where density difference is a problem, it is preferable to use the gas mixture immediately after mixing. Specialized mixing apparatus may be required when one of the gases in the mixture is present in a relatively low concentration, increasing the difficulty of preparing a homogeneous mixture. For some applications, the gas mixing apparatus can have moving internal parts or stationary internal parts which assist in the mixing of the gases. However, for applications in which contamination of the gas mixture due to the erosion or corrosion of such internal parts is a critical factor, it may be necessary to avoid the presence of such internal parts. Further, internal parts may also provide a corner, crevice or dead space which permits particle accumulation.
Chen et al., in U.S. Pat. No. 5,113,028, issued May 12, 1992, describe a process for mixing hot ethane with chlorine gas using a tubular (pipe) mixer having no internal parts. Ethane gas is conducted through a main pipe, and chlorine gas is introduced into the main pipe through four or more jets. The angle between the axis of each jet and the line from the center point to the point where the axis of each jet makes contact with the inside surface of the main pipe ranges between about 30.degree. to 45.degree. . After the introduction of the chlorine gas, the combination of ethane and chlorine gas travel coaxially through the pipe to complete mixing, with a reaction taking place when the gas mixture reaches an appropriate temperature. The length of the pipe is at least 10times the diameter of the pipe; the ratio of the pipe diameter to the jet diameter ranges from about 21:1to 8:1; the velocity of the gases traveling through the pipe is less than the speed of sound, but such that the Reynolds number for each gas is at least 10,000; and, the ratio of the chlorine gas velocity to the ethane gas velocity ranges from approximately 1.5:1to 3.5:1. The mixer is designed to insure sufficient friction between the gases during mixing that the temperature of the mixture of gases, without any heat due to chemical reaction, reaches a temperature of approximately 225.degree. C. or higher after mixing. It is this latter requirement that determines the relative velocities of the gases passing through the mixer and the requirement that there be at least four jets positioned as described around the circumference of the pipe.
Another gas mixing apparatus having no internal parts which contribute to the mixing is described by Dunster et al. in U.S. Pat. No. 4,865,820, issued Sept. 12, 1989. This apparatus is a combination gas mixing and distribution device. The mixer - distributor is used to feed a gaseous mixture to a hydrocarbon reforming reactor. A principal feature of the apparatus is that the apparatus mixing section provide turbulent gas flow, to ensure substantial mixing of the gases, and that the gas mixture velocity within the apparatus distributor section exceed the flashback velocity of a potential flame from the reaction chamber into the mixing chamber. The gas mixer comprises a plurality of tubes inside a chamber, wherein each tube has a plurality of orifices which communicate with the surrounding chamber. A gas or gaseous mixture flows through the interior of each of the tubes. A second gas or gaseous mixture flows from the surrounding chamber into each tube through the plurality of orifices. As the gas from the surrounding chamber flows into each tube, it mixes with the gas flowing through the tube and this mixture flows into the distributor and from there to the reactor. The size of the internal diameter of the tubes as well as the length of the tubes is designed to produce uniform gas flow through the tubes. The size of the orifices is selected to provide sufficient pressure drop between the surrounding chamber and the tube interior to provide for the desired gas feed rate from the surrounding chamber into the tubes. There is no particular requirement that the orifices be located in a particular position relative to each other. FIGS. 2, 5, and 7 show at least three orifices located around a circumference of each tube. FIG. 2 shows orifices at more than one circumferential location on each tube.
A third mixing apparatus having no internal pans which contribute to the mixing is described by Vollerin et al. in U.S. Pat. No. 4,089,630, issued May 16, 1978. This apparatus mixes two fluids by generating a pressure drop across a pair of surfaces each forming a wall of a mixing chamber and confronting one another, while separating a respective source of fluid from the mixing chamber. The surfaces are provided with mutually aligned and opposing apertures, thereby accelerating the respective gases through the apertures in opposing jets. The resulting mixture of fluids is conducted away from the chamber in a direction substantially parallel to the surfaces. In particular, this mixing apparatus was designed for mixing of a recirculated combustion gas and a combustion-sustaining gas such as air for combustion of the mixture with a combustible gas.
All of the above-described gas-mixing devices employ a gas flowing through an orifice to contact and mix with another gas. There are many examples of the use of orifices in the mixing gases and fluids in general, including a multitude of examples pertaining to carburation. In each case, the apparatus design depends on the end use application and the tasks to be accomplished by the apparatus.
The gas mixing apparatus and method of the present invention was developed for use in the semiconductor industry where it is often desired to create a gas mixture including a very small quantity (parts per million or less) of one component gas, such as a dopant gas. In addition, in many circumstances the gases to be mixed have substantially different densities.
The apparatus used to provide the gas mixture must not contribute particulate contamination to the gas mixture, since it is critical that gases used in semiconductor production have extremely low particulate levels. The presence of particulate contamination can render inoperable a semiconductor device having submicron-sized features. Previously utilized gas mixing apparatus having internal static mixer configurations have not proved satisfactory, due to the generation of particulates. To avoid the generation of particulates, it is helpful that the gas mixing apparatus be free from internal parts which contaminate the gas mixture due to erosion or corrosion of such internal parts.
Many of the dopant gas mixtures used in the semiconductor industry contain dopant constituents at concentrations in the parts per million (ppm) or parts per billion (ppb) range. Further, the dopant constituent typically has a significantly different density from the diluent carrier gas used to transport it into the semiconductor process. Since it is critical to the performance properties of the semiconductor device that the dopant be present at a specified concentration and that it be uniformly distributed, the dopant gas used to supply the dopant must be homogeneous and have proper dopant content. Thus, it is frequently preferred to mix the dopant gas into the diluent carrier gas immediately before use. Further, since some of the dopant constituents are relatively toxic, it is not desirable to mix large quantities of the component gases to obtain a uniform mixture, with excess gas mixture to be discarded; it is preferred to mix small quantities of gas as required for use. Due to the desire to produce small quantities of homogeneous dopant gas mixtures, it is important to have highly turbulent mixing, so that a uniform, homogeneous gas mixture can be obtained rapidly upon contact of the gases to be mixed, even when the relative quantity of one of the gas constituents is small.
The above-described specialized requirements have created a need in the semiconductor industry for a gas mixing apparatus and method which provide for highly turbulent mixing of small quantities of gases, with mixing achieved in an apparatus having minimal to no internal parts to contribute to the generation of particulates.